As a conventional example position detection system that makes use of an ultrasonic wave, an electronic pen system is disclosed in Patent Document 1. This example position detection system includes an electronic pen and a receiver section. The electronic pen transmits an ultrasonic wave signal of a constant waveform at a fixed period and an infrared trigger signal at a fixed period, and the receiver section receives the two signals transmitted. The receiver section determines the position of the electronic pen from an incoming time point of the trigger signal and that of the ultrasonic wave.